Electrolytic production of oxygen difluoride



United States Patent 3,276,981 ELECTROLYTIC PRODUCTION OF OXYGEN DIFLUORIDE John A. Donohue, Chicago, 111., and William A. Wilson,

Griflith, Ind., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana No Drawing. Filed Nov. 5, 1963, Ser. No. 321,407 Claims. (Cl. 204-101) This application is a continuation-in-part of our copending application S.N. 141,177, filed September 27, 1961, now abandoned. This invention relates to the electrolytic productfon of mixtures of ozone and oxygen difluoride and particularly Where oxygen difluoride is the predominant product and fluorine is produced only in insignificant amounts, if at all.

An electrolytic method of generating oxygen difluoride has been discovered wherein an electric current is passed through a liquid electrolyte consisting essentially of hydrogen fluoride, dissolved inorganic fluoride, and about 0.05-2 0 mole percent of water to produce a mixture of gases containing oxygen difluoride; oxzone and oxygen are also produced at the anode. The amount of oxygen difluoride produced is substantially greater than the amount of ozone produced. Desirably the electrolyte consists essentially of HF, dissolved alkali metal fluoride and about 0.1-1.0 mole percent of water.

The electrolytic method of the invention can be carried out in any electrolytic cell wherein a liquid electrolyte can be positioned in the cell and an electric current passed therethrough; more commonly, electrodes are immersed in the liquid electrolyte and provisions are made for maintaining liquid electrolyte at the desired temperature of operation. Electrolytic cells suitable for the carrying out of the method of the invention include those now used for the electrolytic production of fluorine except that the carbon anode needs to be replaced by a metal anode. Illustrative descriptions of suitable cells are given in chapter 8, Fluorine Chemistry, J. H. Simons, editor (Academic Press, 1950). It is to be understood that the cells will be modified by anyone of ordinary skill to materials of construction suitable for use with the particular electrolytes used herein. It has been found that particularly suitable materials of construction for the electrodes are: anode formed of nickel and cathode formed of iron, specifically, black iron.

When an electric current is passed through a liquid electrolyte consisting of hydrogen fluoride, dissolved inorganic fluoride and about ODS-2.0 mole percent of water, gases are produced, These gases include hydrogen, molecular oxygen, oxygen difluoride (0P and ozone (0 The hydrogen is produced at the cathode, the other gases being anode products.

The presence of hydrogen in the product gases may be drastically reduced or essentially eliminated by bathing the cathode with oxygen which reacts with the hydrogen produced at the cathode to form water. This provides make-up water in the electrolyte and increases safety. An especially convenient method of doing this is to form the cathode of porous metal and pass the oxygen through it. By using a porous cathode the oxygen is highly dispersed and a large cathode surface is provided so that the reaction of oxygen with the hydrogen is enhanced. Other suitable methods for providing oxygen at or near the cathode for reaction with the hydrogen can be devised by those skilled in the art. The water produced dissolves in the electrolyte. Under most operating conditions using oxygen at the cathode the water concentration in the electrolyte will tend to build up, because it is formed at a higher rate than it is consumed, so that it will be necessary to remove water in order to maintain 3,276,981 Patented Oct. 4, 1966 the desired concentration. A convenient source of oxygen is that produced at the anode of the cell and separated from the other anode products.

It is to be understood that hydrogen fluoride (HF) as used herein may include regular commercial grade acid as well as high purity hydrogen fluoride itself. When utilizing commercial grade acid, the Water content thereof will be calculated as part of the total desired water content of the particular liquid electrolyte present in the electrolytic cell.

In addition to hydrogen fluoride and water the electrolyte used in the method of the invention contains HF- soluble inorganic fluoride. The inorganic fluoride must have a suificient solubility in liquid hydogen fluoride to pass the electrolytic current in a significant amount. When the current is passed through an electrolyte consisting only of hydrogen fluoride and water, ozone is normally produced in larger amounts than oxygen difluoride is produced. However, when an inorganic fluoride is present, dissolved in the electrolyte, the oxygen difluoride is produced in a greater amount than is ozone, in the ranges of electrolyte compos'tion of most interest to commercial operation. At certain combinations of hydrogen fluoride, dissolved inorganic fluoride and Water the production of oxygen difluoride and ozone is more or less equal; in general the production of oxygen difluoride is substantially greater than the production of ozone.

The inorganic fluorides are desirably metal fluorides Where the metal component is positioned above hydrogen in the electromotive series of metals. A listing of these is presented at page 686, Langs Handbook of Chemistry (1934). The alkali metal fluorides are particularly suitable inorganic fluorides for use in the method of the invention. Potassium fluoride, sodium fluoride and lithium fluoride are preferred.

Suflicient inorganic fluoride must be present in the electrolyte to shift the production of oxygen difluoride towarde the predominant position relative to ozone. More than this amount may be used. The maximum amount of inorganic fluoride is determined in part by the water con tent of the electrolyte and the temperature of operation for the particular cell, and keeping the production of fluorine to either none or an insignificant amount. When it is desired to maximize the production of oxygen difluoride relative to ozone, it is preferred to use alkali metal fluoride in the electrolyte and in an amount of 01-10 :mole percent of the total electrolyte.

Some water must be present in the electrolyte in order to keep the production of fluorine at least at an insignificant amount; normally no fluorine is produced. Thus the amount of water present in the electrolyte is related to the desired production of only insignificant amounts of fluorine or none at all, and the desired production of ozone along with the oxygen difluoride. The water in the electrolyte is a convenient source of oxygen for the production of ozone and oxygen difluoride. Broadly, the Water content of the electrolyte is about 0.052.0 mole percent of the total electrolyte. Usually the Water content of the electrolyte is about 0.1-1.0 mole percent. When it is desired to maximize the production of oxygen difluoride relative to the production of ozone, using 0.2-2.0 mole percent alkali metal fluoride in the electrolyte, the preferred water content of the electrolyte is about 0.2-0.5 mole percent.

It is to be understood that the electrolyte used in the method of the invention is used as the basis of determining the mole percentages set forth above and that the hydrogen fluoride portion makes us essentially the remaining amount after the inorganic fluoride and water contents have been specified.

The electrolyte must be in the liquid state and suflicient spressure must be maintained on the cell to keep the elec- :J trolyte in the liquid state at the particular temperature of operation.

The electrolytic cell is operated at any temperature which will permit the production of oxygen difluoride and produce no or only insignificant amounts of fluorine (F In general, the cell is operated at a temperature of not more than about 200 C. and desirably at a temperature of not more than about 100 C. More commonly, the cell is operated at a temperature from about -30 C. to +50 C.

The product of the electrolysis comprises a mixture of hydrogen, oxygen, oxygen difluoride and ozone. (Fluorine is either not present or produced in an insignificant amount.) The hydrogen may be readily separated from the other gaseous products by condensingthese three.

The oxygen and ozone may be removed by low temperature distillation from the oxygen difluoride. It has been discovered that silica gel adsorbs ozone in preference to oxygen difluoride and essentially pure oxygen difluoride may be recovered as eflluent from the silica gel adsorption zone.

Illustrations The cell used in this work was composed of a Kel-F cup covered by a stainless steel cap. The use of Kel-F allowed the contents of the cell to be observed during the electrolysis. The cup was approximately 2". in diameter and 4" high.

No cell diaphragm was used since earlier work showed that separation of the anode and cathode compartments was unnecessary. Two nickel anodes and three iron cathodes, 3 x 1%, made up the electrode pack. Teflon spacers CA) were used, and the pack was held together by steel bolts insulated with Teflon sleeves. The electrolyte was cooled by an external ice bath and a copper cooling tube placed in the electrolyte. Acetone from a Dry- Ice bath (-78 C.) was circulated through this tube to regulate the electrolyte temperature. The coolant flow was regulated by a temperature controller connected to a thermocouple in the electrolyte. Both the thermocouple well and the coolant coil were coated with Kel-F wax to prevent corrosion.

Maximum P yields were obtained at conditions where F and 0 were also generated. Concentrations of these products were determined by absorbing the ozone on silica gel and analyzing the 0P and F the ozone was then desorbed and analyzed.

For this analytical procedure, cell gases were passed through a tube, containing silica gel at 78' C. and into a 2% KI solution in a gas bubbler bottle. Ozone was absorbed from the product stream and DE and F reacted with the K1 solution to liberate iodine. The KI solution was then analyzed for iodine and fluoride ion. If only 0P was present, the normality of iodine was just twice that of fluoride ion. However, if fluorine was present, the ratio was less than 2:1 and both F and 0P concentrations were calculated. Then a second KI solution was attached to the silica gel tube, and ozone was flushed off the silica gel by gentle warming in a nitrogen stream. Care was taken not to warm the gel too rapidly lest some ozone be decomposed. Analysis of the second KI solution for iodine then gave the 0 yield. Repeated checks for fluoride ion in this second KI solution have shown insignificant amounts to be present. Therefore, little, if any, F or 0P was adsorbed with the 0 on the silica gel.

The water content of the electrolyte was checked by a Karl-Fischer titration after every two or three runs.

Illustrative experimental results are shown in the table. The data show that, surprisingly, the 0P yield rises rapidly and falls off sharply as water concentration changes by a few tenths of a percent. As the concentration of alkali metal fluoride, NaF or KF, increases, the maximum 0P yield occurs at a higher water concentration, and appears to decrease. The F and 0 yields are also affected by alkali metal fluoride concentration. As alkali metal fluoride concentration was increased, yield of O decreased while F formation became difficult to suppress and persisted out to higher water concentrations.

Voltage drop between the anode and cathode outside the range 7.07.8 gave lower yields of CE. At high alkali metal fluoride and low water concentrations, increasing voltage above 7.8 resulted in larger amounts of F being generated. When the voltage was decreased below 7.0, the formation of 0 became the predominant reaction.

TABLE Added 1 Product Yicld Test No.

NaF KF H2O Volts Amps OF: 0 F1 0 0. 6 0 0 0 75 0 0. 6 O. 18 8. 3 7. 7 56 5 9 0 0.6 0.26 7. 0 4. 9 64 10 2 0 0. 6 0. 32 7. 0 4. 6 57 8 0 0 0.6 0.55 7. 4 7.0 41 20 0 0 0.6 0. 7.8 7. 8 33 28 0 0 1. 5 0. 30 7. 4 5. 5 55 6 5 0 2. l 0. 38 7. 5 6. 5 61 7 0 0 1. 5 0.50 7. 8 9.0 53 12 0 0 1. 5 0.66 7. 8 8. 3 36 18 0 0 3. 0 0. 50 7. 8 7. 8 56 5 5 0. 6 0 0. 25 7. 0 2. 4 22 9 trace 0. 6 0 0.58 7. 0 1. 8 36 10 0 0. 6 0 1. 3 7. 0 1. 8 21 17 0 5. 1 0 1. 2 7. 0 1. 6 21 6 0 4. 9 0 5. 3 6. 4 0. 6 l4 l4 0 l Mole percent on total electrolyte, remainder essentially HF.

2 Averaged over the test because of small variations.

3 Hydrogen and oxygen also produced.

4 Yield expressed as the percent of the current passed in the cell which is accounted for by the amount of product.

Anode corrosion has not been studied directly but qualitative observations were made. One cell wasused to prepare 0P intermittently for over hours. Variation of water concentration in the electrolyte sometimes resulted in high yields 15%) of both 0 and F Thus, the anode'was used at a wide variety of stringent conditions. .Yet high yields of 0P are much less efl'ected by anode corrosion than 0 yields. When a cell is to be operated intermittently, it is preferred that ,both electrodes be nickel because iron corrodes while the cell is inactive.

Oxygen difluoride is a powerful oxidizer and may be used for treating waste organic matter where no hazard to animal life is present. Another use for this compound is as a catalyst in the increasingly important field of fluorinated polymers, such, polytetrafluoroethylene production (US. Patent No. 2,757,167).

Thus having described the invention, what is claimed is:

v1. A process for producing oxygen difluoride from hydrogen fluoride and water which process comprises. passing an electrical current through a liquid electrolyte positioned in an electrolytic cell, said liquid electrolyte consisting essentially of hydrogen fluoride, alkali metal fluoride in the range of about 0.2-2.0 mole percent, and about 0.101.0 mole percent of water, and withdrawing from said cell gases containing oxygen difluoride.

2. The process of claim 1 wherein said electrolyte is maintained at a temperature below about 200 C.

=3. A process for producing oxygen difluoride electrolytically from hydrogen fluoride and water which process comprises electrolyzing an electrolyte consisting essentially of hydrogen fluoride, alkali metal fluoride in the range of about 02 2.0 mole percent, and about 0.2-1.0 mole percent water in an electrolytic cell containing an anode and a cathode, said electrolyte being maintained at a temperature below about 200 C. and recovering oxygen difluoride from the anode gases produced in said cell.

4. In a process for producing oxygen difluoride by the electrochemical fluorination of water, the method of producing oxygen difluoride as the major anode gas which 1 comprises electrolyzing a liquid electrolyte in an electrolysis cell containing an anode and a cathode, said elec-. trolyte being maintained at. a temperature below about- 200 C., said electrolyte comprising at least about 90 mole percent hydrogen fluoride, an alkali metal fluoride in the range of about 09 -2.0 mole percent, and 0.1-1.0 mole percent water, and withdrawing as product gases produced in said cell in which product oxygen difiuoride is the major gas produced at said anode.

5. A process for .producing oxygen difluoride from hydrogen fluoride and water which process comprises passing an electrical current through a liquid electrolyte positioned in an electrolytic cell, said electrolyte being maintained at a temperature below about 200 C., said electrolyte consisting essentially of hydrogen fluoride, alkali metal fluoride in the range of about 0.22.0 mole percent, and Water, controlling the concentration of water in said electrolyte in the range of 0.2-1.0 mole percent, based on electrolyte, so that oxygen difluoride is the major anode product, and withdrawing oxygen di fluoride from said cell.

References Cited by the Examiner UNITED STATES PATENTS 2,034,458 3/1936 Calcott et al 204128 XR 5 OTHER REFERENCES Chemical Abstracts 1, volume 22, page 200, 1928. Chemical Abstracts II, volume 54, column 13,900e, 1960.

Engel brecht et al.: Uber cine electrochemische Darstel- 10 lung Von Sauerstoif-difluoride, Monatshefte Fur Chemie,

pages 368 to 370, vol. 90, 1959.

JOHN H. MACK, Primary Examiner.

15 H. M. FLOURNOY, Assistant Examiner. 

1. A PROCESS FOR PRODUCING OXYGEN DIFLUORIDE FROM HYDROGEN FLOURIDE AND WATER WHICH PROCESS COMPRISES PASSING AN ELECTRICAL CURRENT THROUGH A LIQUID ELECTROLYTE POSITIONED IN AN ELECTROLYTIC CELL, SAID LIQUID ELECTROLYTE CONSISTING ESSENTIALLY OF HYDROGEN FLUORIDE, ALKALI METAL FLUORIDE IN THE RANGE OF ABOUT 0.2-2.0 MOLE PERCENT, AND ABOUT 0.10-1.0 MOLE PERCENT OF WATER, AND WITHDRAWING FROM SAID CELL GASES CONTAINING OXYGEN DIFLUORIDE. 